Non-inflatable roller elements or wheel assemblies conventionally are fabricated with some sort of bearing hub about which is positioned and/or bonded a tread layer of more resilient material than the bearing hub. Examples of various applications for such wheel assemblies include high performance tires for fork trucks, dollies, street sweepers and other heavy load tires, as well as drum and cylinder rollers, conveyor rollers or the like. Such wheel assemblies must withstand a wide variety of loads, temperatures, chemicals and surface conditions.
Heretofore, the most common wheel assemblies for such usages have been made with a tread layer of rubber or polyurethane. Unfortunately, such materials do not provide sufficient resistance to abrasion, cuts and/or cut propagation. Such materials also have high failure rates due to hysteretic heating which softens the material, releases the bond between the material and the bearing hub, and simply breaks down the material. Such materials also lack sufficient contact and impact load capacity. In fact, when using fork lift trucks in high activity installations, where the wheel or tire tread layer is fabricated of polyurethane material, the wheel assemblies may have to be replaced as often as every other day.
This invention is directed to solving these problems by providing a remarkably new and improved non-inflatable wheel assembly which has greater material memory, very low hysteretic heating, very high contact and impact load capacity and has excellent resistance to abrasion and/or cut propagation. These advantages are provided by a new and improved composition of the tread layer, as well as a method and apparatus for manufacturing a wheel assembly incorporating the new composition tread layer.
More particularly, copolyester polymer elastomer material was discovered and disclosed in U.S. Pat. No. 4,198,037 to Anderson, dated Apr. 15, 1980 and assigned to the assignee of this invention, for use as a novel elastomeric spring. As disclosed therein, one such polymer, sold under the trademark HYTREL, is made from three basic ingredients, namely, (1) dimethyl terephthalate, (2) polyglycols such as polytetramethylene ether glycol, polyethylene ether glycol or polypropylene ether glycol, and (3) short chain diols like butanediol and ethylene glycol.
Such copolyester polymers act suitably when employed as tension springs. However, they have proven to be quite unsuitable for use as a compressive spring material. This is due to the fact that when compressed over about ten percent they commence taking on a permanent set. Obviously, a resilient material that permanently deforms under load is quite unsuitable for the intended purposes.
However, the patent described above discloses a method by which a thermoplastic elastomeric material, such as the copolyester polymer elastomer material, can be treated for rendering the material usable as a resilient material, particularly a compression spring. Generally, that treatment, to convert the elastomer into a resilient or compression spring material, comprises the application of a compressive force to a body or thickness of the material which compresses the body or thickness to an extent equal to at least thirty percent of its previous free thickness, measured in the direction of the applied pressure.
This invention incorporates such material in a new and improved wheel assembly by a novel method and apparatus to produce a rolling element or tire having remarkable properties not heretofore available.